Fix style sheet parsing

This is for #9875 .

When minimizing style sheets for parsing, limit string removal to content property.

Added unit tests to CSSUtils and Quick View.

This API call was added specifically for Quick View, and Quick View is still the only user of this API. The original intent was to remove content property strings that would prevent us from detecting unbalanced parens (which cause regexp to crash). I think renaming this function accordingly would properly describe its intent. Agreed?